Method of and apparatus for generating ultra-short time-duration laser pulses

ABSTRACT

Both a method and an apparatus are disclosed for generating laser pulses having a time duration on the order of subnanoseconds. These light pulses also have a high output power. They are generated by causing a laser to lase and removing the bulk of the radiation contained in the laser cavity. The remaining radiation or light extends over only a short length of the cavity. This light pulse is mode locked, that is it consists of individual Fourier components of the proper phase so that the short light pulse retains its shape while being amplified again in the laser cavity. This short-duration light pulse may then be made to issue from the cavity. Alternatively, the amplified light may be made to issue as a pulse train or set of pulses. Finally, a single pulse may be utilized for mode locking another laser where the ultra-short time-duration pulse may be amplified again. This may be considered priming and subsequent mode locking of the second laser.

United States Patent 1 Simmons 51 May 29,1973

[75] Inventor:

William W. Simmons, Peninsula, Calif.

[73] Assignee: TRW lnc., Redondo Beach, Calif.

[22] Filed: Aug. 7, 1972 [21] Appl. No.: 278,373

Related US. Application Data [62] Division of Ser. No. 143,515, May 14,1971, Pat. No.

OTHER PUBLICATIONS Vinogin et al., Narrow Line Ruby Laser. Optics andVoltoqe Generotor Spectroscopy (USA) Vol. 28, No. 1 (Jan. 1970) pp. 85and 86 Primary aemin rrzw l i ksim Attorney-Daniel T. Anderson, Edwin A.Oser and Jerry A. Dinardo [57] ABSTRACT Both a method and an apparatusare disclosed for generating laser pulses having a time duration on theorder of subnanoseconds. These light pulses also have a high outputpower. They are generated by causing a laser to lase and removing thebulk of the radiation contained in the laser cavity. The remainingradiation or light extends over only a short length of the cavity. Thislight pulse is mode locked, that is it consists of individual Fouriercomponents of the proper phase so that the short light pulse retains itsshape while being amplified again in the laser cavity. Thisshort-duration light pulse may then be made to issue from the cavity.Alternatively, the amplified light may be made to issue as a pulse trainor set of pulses. Finally, a single pulse may be utilized for modelocking another laser where the ultra-short time-duration pulse may beamplified again. This may be considered priming and subsequent modelocking of the second laser.

3 Claims, 7 Drawing Figures 1 Voltage Generator Patented May 29, 19733,736,526

' 2 Sheets-Sheet z 38 l I I Kerr Cell Pulse Retardation Mt) I I I 42I I\L. Optical Power At Arrow 25 2L I l 1 l l W l i" I Optical Power InCavity Incident On Mirror I6 53 1 CAV I 43 I We I 44 2L A 0%+r%+r%+r%+r% "If'IEIC-"I TIME- 9 Hg 3 T 0 2L K VOI'S I '5' P OpticalPower w K Volts l 5*] K Valte W K Volt:

84 as I P Optical Power 90 8 I Optical Power Output Time BACKGROUND OFTHE INVENTION This invention relates generally to lasers, andparticularly to a laser so operated as to generate a pulse having asubnanosecond time duration.

For many applications it is desirable to operate a laser in such amanner that it generates output pulses The radiation remaining in thecavity is now amplified of large energy having a time'duration on theorder of nanoseconds or even less. However, in the past it has beendifficult, if not impossible, to generate a light pulse having aduration in the subnanosecond region which also has a high output power.

It is known to operate lasers by time-variable reflectivity. Such alaser has been disclosed and claimed in a US. Pat. No. 3,571,744whichissued on Mar. 23, 1971 to Hook and Dishington and is entitledLaser Incorporating Time Variable Reflectivity. This patent is assignedto the assignee of the present application. The laser is initially madeto lase in a conventional manner. Subsequently, by means of Q switchingall the light contained in thelaser cavity is reflected or refracted outof the laser cavity. This may, for example, be effected'by changing thepolarization of the light in such a manner that all the light containedinthe cavity is switched out of the cavity by a birefringent prism orthe like. In that case, obviously the time duration of the light pulsecan be no less than the time duration of the light passing from one endof the cavity to the other and back again to its'origin, that is thetime it takes light to pass twice the length of the cavity.

Thus the time duration of such a light pulse may be in the order of afew nanoseconds and its amplitude or energy is relatively high; However,it is not possible'to reduce the time duration of the light pulseobtained with such a time-variable reflectivity laser.

Another scheme for generating giant laser pulses of short time durationhas been disclosed in a patent to Witte and Frantz U.S. Pat. No.3,506,927 and entitled Selected Mode Giant Pulse Laser." This patent isalso assigned to the assignee of the present invention. It is proposedhere to inject a radiation signal or pulse from a first laser into asecond-laserthereby to amplify only oscillations of a desired modegroup. Accordingly the Witte and Franz giant pulse laser operates byinjection locking of the power laser.

It is accordingly an object of the present invention to provide a methodof and apparatus for generating laser pulses having a time duration onthe order of subnanoseconds.

Another object of the present invention is to provide 7 a practicalmethod of injection locking of a laser for the purpose of generatingultra-short time-duration light pulses of high power.

A further object of the present invention is to generate an amplifiedlight pulse in the laser cavity which is short compared to the length ofthe laser cavity by mode locking of the laser.

provided of operating a laser to generate a pulse of radiation having anultra-short time duration. To this end the laser which has an opticalcavity is caused to lase until the cavity is substantially filled withradiation. Thereafter the bulk of the radiation contained in the cavityis removed. As a result radiation contained in the cavity is removed. Asa result radiation remaining in the cavity extends only over a shortlength of the cavity.

by laser action to develop an amplified, short-duration pulse. This ismade possible because the pulse has Fourier components which match theFabry-Perot resonances of the cavity. Thus the short time pulse may beconsidered to consist of radiation components extending over a certainfrequency range and having such phase relationships that the laser ismode locked. Therefore, the pulse remains together while it is reflectedback and forth between the mirrors defining the cavity of the laser andwhile the pulse is being amplified. Finally, the short pulse which is ata high power level is utilized.

This may be effected by causing the short-duration pulse to issue fromthe cavity. Alternatively it is feasible to issue a pulse trainconsisting of a series of amplified pulses. Finally the short durationpulse may be utilized for triggering or priming another laser, that is,for injection locking the second laser. The second laser may be arrangedto have high amplification to produce an output pulse with the sameshort duration but having even a higher energy level. i

I The novel features that are considered characteristic ofthis'invention are set forth with particularity in the appended claims.The invention itself, however, both as to its organization and method ofoperation, as well as additional objects and advantages thereof, willbest be understood from the following description when read inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS V FIG. 1 is a schematicrepresentation, partly in block form, of a laser system in accordancewith the present invention for generating an ultra-short time-durationpulse;

FIG. 2 is anend elevational view of a Kerr cell which may be used withthe laser FIG. 1;

FIG. 3 is a chart of a series of pulses representing light waveretardation and optical power in the'cavity' of the'laser of FIG. 1 as afunction of time;

FIG. 4 is a circuit diagram, partly in block'form, illustrating by wayof example a voltage generator for generating very short time durationpulses which may be used with the laser of FIG. 1;

FIG. 5 is a chart illustrating the'optical power as a function of timewithin the cavity of a laser when the laser is operated in a particularway to change the relative mode phasing of the light remaining in thecavity;

FIG. 6 is a schematic representation, partlyin block form, of a modifiedlaser system in accordance with the present invention for injecting alight pulse generated in a first laser'into a second laser for furtheramplification of the mode-locked pulse; and

FIG. 7 is a set of curves plotted as a function of time to illustratethe voltages applied to various Kerr cells of the system of FIG. 6 aswell as the optical power remaining in the two laser cavities of thesystem'of FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings,and particularly to FIG. 1, there is illustrated a laser system forgenerating high-power, ultra-short time-duration pulses in accordancewith the present invention. The laser system of FIG. 1 includes a lasergenerally indicated at 10. The laser may, for example, be a gaseouslaser as shown and may be provided with a pair of windows 11 and 12arranged at the Brewster angle to minimize reflection of the laserlight. Also the light passing through the Brewster windows 11 or 12 willbe linearly polarized. However, it should be understood that anyconventional laser may be substituted for the gaseous laser such, forexample, as a solid state laser. The laser is provided with a pumpingpower source 14 which may be connected for example in the case of agaseous laser to a pair of electrodes 15 immersed in the gas. However,it will be understood that in the case of a solid state laser, thepumping power source 14 may energize, for example, a flash lamp foroptical pumping of the laser. I The optical cavity is defined by a pairof mirrors l6 and 17. The mirrors may, for example, be spherical mirrorsas shown which facilitates adjustment of the cavity. However, it isfeasible instead to use mirrors with plane surfaces. Preferably the twomirrors 16 and 17 are coated at their opposed surfaces. This may, forexample, be effected by a suitable dielectric coating. In general, themirrors l6 and 17 should be made completely reflecting. In practice theymay have a reflectivity of about 99 percent.

Further disposed in the optical cavity is an electrooptical element 18which may, for example, consist of a Kerr cell as shown or alternativelyof a Pockels cell or of an acoustic cell. In general any electro-opticaldevice may be used which is capable of changing the polarization oflight passing therethrough in response to an applied voltage or the likesignal. Thus the electrooptical element 18 such as a Kerr cell may beprovided with two electrodes 20 and 21 one of which is grounded whilethe other one is connected to a voltage generator 22. The voltagedeveloped by the generator 22 will be discussed hereinafter inconnection with FIG. 3.

Further disposed in the optical cavity is a birefringent element 24which may, for example, consist of a calcite prism as shown. However,any birefringent element 'may be substituted therefor. It has thepurpose of refracting the light passing therethrough selectively eitherbetween mirrors l6 and 17 or in the direction of arrows 25 or 26depending on the polarization of the light passing therethrough. To thisend it must be assumed that the light reflected between mirrors 16 and17 has a normal direction of polarization. If the light issued by thelaser 10 is not normally so polarized this may be effected by a suitablepolarizer interposed, for example, between laser 10 and Kerr cell 18.-However, the light passing a laser having windows 11 and 12 disposed atthe Brewster angle will automatically be linearly polarized.

It will be noted that the calcite prism 24 refracts the light so that acavity is used consisting of two branches disposed substantially atright angles to each other with the prism 24 at the intersection of thetwo cavities. However, this particular arrangement of the optical cavityis not essential for the operation of a laser system in accordance withthe present invention. Thus the laser of FIG. 6 which will be discussedlater on consists of two optical cavities each being disposed along asingle line and intersecting each other at right angles.

The laser system of FIG. 1 may optionally be provided with a secondelectro-optical element 27 which may be identical with the element 18and may consist of a Kerr cell as shown. It also has a pair ofelectrodes 28 and 30 one of which is grounded as shown while the otherone is connected to a voltage generator 31 which may be similar to thevoltage generator 22.

Thus with the normal direction of polarization of the light, the lightwill move in the direction shown by the dotted line 32 through the prism24 as shown by line 33 and will be refracted out of the prism as shownby the line 34. The laser is permitted to lase by energizing it throughthe pumping power source 14. This is continued until the entire lasingcavity is filled with light radiation reflected back and forth betweenmirrors 16 and 17. Thus the light power builds up to some steady-stateoptical radiation level. This may be denoted by the lasing energy W andit may be assumed that this energy is uniformly distributed throughoutthe cavity. The cavity may have a length L so that the optical powerflow in one direction is as follows: Wc/ZL, where c is the velocity oflight.

After this steady state has been reached a voltage pulse V(t) is appliedby the voltage generator 22 to the Kerr cell 18. This voltage pulseshould have a very fast rise time. This is more clearly shown in FIG. 2which may be considered to be an end view of the Kerr cell 18 so thatthe light passes through the cell 18 in a direction at right angles tothe paper plane. The normal polarization direction is shown in FIG. 2 bythe arrow. Accordingly the voltage V(t) creates a new polarizationcomponent as shown by the dotted arrow 36. This polarization componentis in the direction of the horizontal plane. It will be assumed for thefollowing discussion that the light passes twice through the Kerr cell18. To this end the Kerr cell 18 may be disposed close to one of themirrors such as 17. A fractional component of the light in thehorizontal direction may be represented as follows: A sin(/2), where d:is the phase delay introduced by the voltage applied to the Kerr cell18. This phase delay or retardation of the light due'to the Kerr cell isshown in FIG. 3 at 38. As shown here it preferably has a steep rise andfalltime and is substantially flat topped. It also has a very short timeduration as will be more fully explained hereinafter.

Due to the application of the voltage pulse to the Kerr cell the lightwave passing through the Kerr cell 18 is retarded as shown at 38 in FIG.3. This in turn will change the polarization of the light so that thelight passing through the prism 24 is refracted as shown by the dottedline 40 and emerges as shown in the direction of arrow 25.

If all the light contained in the optical cavity is removed by the pulseapplied to the Kerr cell 18 the operation is like that of a conventionaltime variable reflectivity laser as disclosed in the previously referredto patent to Hook and Dishington. However, in accordance with thepresent invention not all of the radiant energy contained in the cavityis removed or dumped. Thus the electrical pulse applied to the Kerr cell18 should be sufficiently short so that radiant energy in a localizedportion of the cavity remains in the cavity. What is desired is that theremaining light in the cavity is localized in the form of an opticalpulse. It occupies a region in the cavity having a length of c(-r 'r')L, where 'r is the rise or fall time of the electrical pulse V(t).Further r 1' is the time duration of the light pulse 1- r E 2L/c'1/N,where N is the total number of modes of the light pulse.

This remaining light pulse now passes back and forth by reflectionbetween the mirrors l6 and 17. During this time, of course, the laser iscontinuously pumped by the pumping power source 14. As a result thecirculating pulse is amplified every time it passes through the laser10. After a number of passes of the pulse it has again built up to itsmaximum power level as the laser approaches again its steady-stateoperating condition. It may now be assumed that the laser has againarrived at an energy content W and the peak power of the optical pulseis given by the following relationship W/(r 1') Wc/2L.

Essentially the energy of the amplified pulse is substantially identicalwith the energy of all the radiation initially contained in the lasercavity.

Rerring again to FIG. 3, curve 42 illustrates the optical power at thearrow 25 which is removed from the laser system. The curve 43 of FIG. 3illustrates the power remaining in the cavity and incident on mirror 16.The subsequent pulses shown generally at 44 illustrate the successivelyamplified pulses which are reflected back and forth between mirror 16and 17. As also shown in FIG. 3 the time duration of each of the lightpulses is 'r 1'. It should also be noted that their amplitudes increasemuch beyond the amplitude curve of 43 until they reach the level of therelationship (1). The light pulses follow each other at time intervalsof 2L/c.

Relationship (1) is strictly true only if the laser functions as adistortion-free amplifier while the amplitude of the pulses increase asshown in 44. Actual experiments have shown that the optical pulsespreads somewhat as the laser approaches saturation or its steadystatecondition.

It should be noted that the time duration of pulse 42 is on the order of3 to 4 nanoseconds (nsec). This is the usual limit of an output pulseobtained from a time variable reflectivity laser. On the other hand thetime duration corresponding to 1' 'r' is on the order of 0.3 nsec for anargon ion laser which is at least an order of magnitude shorter induration.

In general the peak power P, of the pulse remaining in the cavity isgiven by the following formula:

and the energy content AW of the pulse is given as follows:

It should be noted that AW should be larger than the spontaneousemission energy of the laser during the pulse duration time. This, ofcourse, is the energy of radiation created by a spontaneous emission inthe laser.

FIG. 4 to which reference is now made shows an example of a voltagegenerator which may be used for generating an electrical pulse of therequired time duration which may be on the order of a few nsec. Thecircuit of FIG. 4 includes a gas filled tube such, for example, as athyratron 46 having its cathode grounded while its control grid isconnected to a trigger signal source 47. The anode may be connected to apositive voltage supply +B through a resistor 48. Further connected tothe anode of the thyratron 46 may be a coaxial transmission line havingits far end shortcircuited and grounded. Such a transmission line willreflect any pulse applied to it within a time period determined by thevelocity of the pulse through the transmission line, and its length.Thus it is possible to develop a pulse having a very short duration bythe use of a short transmission line. The output terminals 51 may beconnected respectively to the anode of thyratron 46 and ground.

As indicated before the short light pulse remaining in accordance withthe present invention in the laser cavity consists of many oscillatinglaser modes of the proper phase with respect to each other. Thus thepulse may have a frequency spectrum extending over a range of a fewgigacycles. This pulse may be considered to have Fourier componentswhich match the Fabry-Perot resonances of the optical cavity formed bythe two mirrors 16 and 17. The amplified pulse such as pulse 53 createdafter sufficient amplification in the optical cavity may then be made toissue from the laser by now energizing the Kerr cell 27 by applyingthereto a short time pulse with the voltage generator 31. This willchange the polarization of the light in such a manner that the lightentering the prism 24 as shown by the dotted line 32 is refracted asshown by the dotted line 54 and emerges in the direction of arrow 26where it may be utilized.

Alternatively it is feasibleto make one of the mirrors 16 'or 17 onlypartially reflective so that it may have a reflectivity say of 50 to 60percent. In that case the light may be made to issue say from the mirror17. In that case instead of obtaining a single light pulse 53 a train oflight pulses is obtained somewhat like the set of pulses shown in FIG. 3at 44. This train of optical pulses may have a fractional nanosecondduration for each pulse and a repetition period of 2L/c which may be onthe order of I00 Mc.

Instead of applying one pulse to the Kerr cell 18 by means of voltagegenerator 22 to initiate the amplification of the short pulses and asecond pulse to the Kerr cell 27 by means of the voltage generator 31 toissue the amplified pulse, it is also feasible to utilize only a singleelectro-optical cell such as a cell 18. In this case two separatevoltage pulses must be applied to the same electro-optical cell, the twopulses being separated and timed properly to permit the shorttime-duration pulse to be amplified sufficiently.

. While it may be difficult to operate a single Kerr cell with therequired double electric pulse by means of thyratrons it is feasible toobtain such an operation, for example, with a Pockels cell and improvedelectronic components such as a KN-6 Krytron tube instead of athyratron.

So far the discussion concerned the localization of the optical energyin as small a space as possible within the optical cavity. This, ofcourse, will achieve maximum output pulse power. However, variation ofthe applied voltage to the Kerr cell 18 changes the relative modephasing of the light remaining in the cavity. This in turn dramaticallyaffects the envelope of the laser recovery. Thus a curve such as shownin 55 in FIG. 5 has been obtained. This depicts the one way opticalpower flow within the cavity as a function of time. The envelope is inthe form of an electronic stairstep wave and rises according to anapproximate exponential. The time duration of each step is again 2L/c. Acurve such as shown in 55 in FIG. 5 may be obtained if enough light isleft within the cavity to fill substantially the entire cavity.

The ultra-short time-duration laser pulse generated in accordance withthe present invention may also be used for injection locking a secondpower laser. Thus a first laser may be utilized to generate the pulsewhile a second laser with a high amplification may be utilized tofurther amplify the pulse. This is generally similar to mode locking ofa laser. The high amplification laser is primed by a short pulse. Thiscan be accomplished with the laser system shown in FIG. 6 to whichreference is now made.

The laser system of FIG. 6 consists essentially of two lasers havingtheir cavities disposed at right angles to each other.

The first laser 60 is enclosed by a pair of mirrors 61 and 62 whichtogether form an optical cavity extending along a straight line. A firstKerr cell 63 designated K, and a second Kerr cell 64 designated K aredisposed in the optical cavity formed by the two mirrors 61 and 62. Abirefringent double prism 65 such as a Glan- Thompson polarizer formsthe intersection between the first and the second laser cavity. Thepurpose of the double prism 65 is to either pass the light straightthrough the prism or to reflect it at right angles depending on thestate of polarization of the light passing therethrough.

The second laser system includes a laser 70 disposed in an opticalcavity formed by mirrors 71 and 72. Again two Kerr cells 73 and 74 aredisposed in the optical cavity and are designated respectively K and KFinally another birefringent double prism or Glan- Thompson polarizer 75is provided in the second laser cavity to permit light to be ejected outof the cavity.

The first laser defined by mirror 61 and 62 may be operated in themanner previously disclosed. Thus the light is initially polarized sayby the Brewster windows of the laser 60 in such a manner that the lightpasses straight through the prism 65. The first Kerr cell K, isinitially energized by a voltage generator 76 to develope a voltagepulse as shown at 77 in FIG. 7. This will change the polarization of thelight in such a manner that the bulk of the light is ejected out of thecavity 61, 62 by the prism 65 into the cavity 71, 72. However, at thattime the second laser 70 is not being pumped so that this light is notbeing amplified. Furthermore, the

second cavity 71, 72 has a low 0 so that its light is ejected out of thecavity by prism 75 as shown by arrow 87.

i As a result, a short pulse remains in the cavity 61, 62. Thisamplified pulse train such as shown in 78 in FIG. 7 is generated in thefirst cavity 61, 62 which is designated as P A sufficiently amplifiedlight pulse is now removed from the first laser cavity 61, 62 byapplying a step voltage 80 to the Kerr cell K;, as shown in FIG. 7. Thisvoltage may be applied by the voltage generator 81 connected to the Kerrcell K,. This will now change the polarization of the light in such amanner that the light is reflected at right angles by prism 65 andinjected into the laser cavity 71, 72.

At the same time the laser cavity 71, 72 is Q-switched by the voltages83 and 84 applied respectively to the Kerr cells K, and K These voltagesmay be generated by the voltage generators 81 and 76 connected to therespective Kerr cells. The voltage 84 applied to the Kerr cell K changesthe polarization of the light to permit it to pass through prism 75between mirrors 71, 72. The voltage pulse 83 changes the polarization ofinjected light to permit it to pass through prism between mirrors 71,72. The laser has previously been pumped to a high inversion of itspopulation just prior to the time T, that is prior to the time of thevoltage pulses 83 and 84.

Accordingly the injected laser pulse is amplified in the optical cavity71, 72 designated P as shown by the pulse train 85 of FIG. 7.

The optical output power is now obtained by removing the voltage thatwas previously applied to the Kerr cell K as shown by the voltage wave84. This will now change the polarization of the light so that the lightis reflected by the prism in the direction of arrow 87. The resultingoutput pulse 88 is also shown in FIG. 7. At this time the voltage wavemay again be removed from the Kerr cell K,. It will be noted that alowamplitude optical pulse 90 is also shown in FIG. 7 as the opticalpower output. This corresponds to the light energy which was initiallydumped from the laser cavity 61, 62 into the laser cavity 71, 72 andwhich appears in the direction of output arrow 87.

There has thus been disclosed a method of and apparatus for generatinglaser pulses having an ultra-short time duration. In spite of theirshort duration, on the order of less than 1 nanosecond, the pulses canbe made to have a substantial output power. It is feasible either togenerate a single pulse or to provide a pulse train of lesser power.Alternatively it is feasible to provide optical pulses which increase inamplitude in the manner of a stairstep wave and which have a timeduration each corresponding to the time it takes light to traverse theoptical cavity twice. Finally the ultrashort duration output pulses maybe used for injection locking a second power laser which in turn mayfurther amplify the original pulse. The advantage of the laser system ofthe invention is that the output pulses have a time duration of lessthan a nanosecond while still having a power or energy corresponding tothat of the pulses obtained, for example, by time variable reflectivity.They may also be used for injection mode locking which is generallyrather difficult to accompplish.

What is claimed is:

1. Apparatus for generating an ultra-short timeduration pulse in a firstlaser and injecting it into a second laser for mode locking said secondlaser, said apparatus comprising:

a. a first optical cavity defined by a first pair of substantiallycompletely reflecting mirrors:

b. a first laser disposed in said first cavity;

c. a second optical cavity defined by a second pair of substantiallycompletely reflecting mirrors, said second cavity intersecting saidfirst cavity;

d. a second laser disposed in said second cavity;

e. means for pumping said lasers to cause them to lase;

f. a first and a second electro-optical element disposed in said firstcavity;

g. a third and a fourth electro-optical element disposed in said secondcavity, each of said elements being capable of changing the polarizationof light passing therethrough in response to an applied voltage;

h. a first birefringent prism disposed in said first cavity and at theintersection of said first and second cavities, said first prism beingnormally arranged i. a second birefringent element disposed in saidsection so as to cause said amplified pulse to be inond cavity, saidsecond prism being normally arjected into said second cavity, and saidvoltage genranged for refracting light having a first predetereratorapplying to said third and fourth elements a mined polarization betweensaid second pair of voltage generator to raise the Q of said secondcavmirrors and being capable of refracting light having ity andpermitting said second cavity to receive and a second predeterminedpolarization out of said amplify said injected pulse and finally toapply a cavity; and

voltage to said second element after the pulse remaining in said firstcavity has been amplified for changing the polarization of the lightpassing therethrough 'into said second predetermined polariza--voltaget'o one of said second and third clements to cause the amplifiedpulse to issue from said second cavity. 2. Apparatus as claimed in claim1 wherein said electrooptical elements are Kerr cells.

3. Apparatus as claimed in'claim 1 wherein said electrooptical elementsare Pockels cells.

j. voltage generators coupled to said elements for applying to saidfirst element a short time duration pulse, whereby the polarization ofthelight passing therethrough is changed from said first to said secondpredetermined polarization so as to cause said first prism to, refractsaid light out of said first cavity and into said second cavity, and forapplying a

1. Apparatus for generating an ultra-short time-duration pulse in afirst laser and injecting it into a second laser for mode locking saidsecond laser, said apparatus comprising: a. a first optical cavitydefined by a first pair of substantially completely reflecting mirrors:b. a first laser disposed in said first cavity; c. a second opticalcavity defined by a second pair of substantially completely reflectingmirrors, said second cavity intersecting said first cavity; d. a secondlaser disposed in said second cavity; e. means for pumping said lasersto cause them to lase; f. a first and a second electro-optical elementdisposed in said first cavity; g. a third and a fourth electro-opticalelement disposed in said second cavity, each of said elements beingcapable of changing the polarization of light passing therethrough inresponse to an applied voltage; h. a first birefringent prism disposedin said first cavity and at the intersection of said first and secondcavities, said first prism being normally arranged for refracting lighthaving a first predetermined polarization between said first pair ofmirrors and being capable of refracting light having a secondpredetermined polarization into said second cavity; i. a secondbirefringent element disposed in said second cavity, said second prismbeing normally arranged for refracting light having a firstpredetermined polarization between said second pair of mirrors and beingcapable of refracting light having a second predetermined polarizationout of said cavity; and j. voltage generators coupled to said elementsfor applying to said first element a short time duration pulse, wherebythe polarization of the light passing therethrough is changed from saidfirst to said second predetermined polarization so as to cause saidfirst prism to refract said light out of said first cavity and into saidsecond cavity, and for applying a voltage to said second element afterthe pulse remaining in said first cavity has been amplified for changingthe polarization of the light passing therethrough into said secondpredetermined polarization so as to cause said amplified pulse to beinjected into said second cavity, and said voltage generator applying tosaid third and fourth elements a voltage so as to raise the Q of saidsecond cavity and permitting said second cavity to receive and amplifysaid injected pulse and finally to apply a voltage to one of said secondand third elements to cause the amplified pulse to issue from saidsecond cavity.
 2. Apparatus as claimed in claim 1 wherein saidelectrooptical elements are Kerr cells.
 3. Apparatus as claimed in claim1 wherein said electrooptical elements are Pockels cells.